This invention relates to pulsed-power excitation networks for optimum power transfer to electric gas laser discharges. The central problems affecting the performance of all high-pressure, self-sustained discharge excited gas lasers are discharge stability and uniformity. In addition, some TE-lasers, such as the rare gas-halide excimer lasers, require very high power densities on the order of 1 MW/cm.sup.3 for effectively pumping the laser. These conditions place very high demands on the PFN used to provide the excitation pulse. In addition to providing the required high voltage and high power levels, it is necessary to use uniform-field electrodes, uniform preionization (by a form of ionizing radiation such as ultraviolet or x-ray radiation) and fast-rising voltage and current pulses having risetimes less than 50 nanoseconds, corresponding to dV/dt and dI/dt values in the range of 10.sup.12 to 10.sup.13 V/s and A/s.
In order to generate these pulses having fast risetimes directly, fast-closing low-inductance switches are required. Known high voltage, high current, fast switches, such as multichannel spark gaps, lack reliability and long life, however. Certain known magnetic pulse compression schemes make it possible to employ slower but more reliable switches, such as thyratrons, thyristors, SCRs, etc. See, for example, U.S. Pat. Ser. Nos. 4,275,317 entitled Pulse Switching for High Energy Lasers and 4,698,518 entitled Magnetically Switched Power Supply System for Lasers. See also D. L. Birx et al., Basic Principles Government the Design of Magnetic Switches, Lawrence livermore National Laboratories, UCID-18831 (November, 1980); I. Smilanski, S. R. Byron, T. R. Burkes, Electrical Excitation of an XeCl Laser Using Magnetic Pulse Compression, Appl. Phys. Lett. 40 (1982) 547; D. Basting et al., Thyratrons with Magnetic Switches: The Key to Reliable Excimer Lasers, Laser and Optoelektronik 2 (1984) 128; and T. Shimada, M. Obara, A. Noguchi, An All Solid-State Magnetic Switching Exciter for Pumping Excimer Lasers, Rev. Sci. Instrum. (November, 1985). In the magnetic pulse compression technique, a microsecond long pulse is first generated with a thyratron-switched circuit. The pulse is then compressed to approximately 100 microseconds by employing two or more compression circuits or stages in tandem. This approach produces sinusoidal voltage and current pulses, which do not permit impedance matching over the entire pulse duration for optimum energy transfer to the discharge plasma.
Since the electric discharge plasma represents approximately a constant voltage load, it is possible to maximize the energy transfer and the laser efficiency by pumping the discharge with a voltage-matched (impedance-matched) constant-voltage PFL. Impedance-matched conditions exist when the open circuit output voltage from the PFL is twice the steady state discharge voltage, V.sub.D. Rapid electrical breakdown of the gas (electron avalanche formation time of less than 20 nanoseconds), however, requires that the applied voltage is three to five times V.sub.D. See D. E. Rothe et al., Efficiency Optimization of a Discharge-Excited XeCl Laser, Final Tech. Report, ONR Contract N00014-82C-0087&lt; Office of Naval Research, Arlington, Virginia (December, 1982) and D. E. Rothe, C. Wallace, T. Petach, Efficiency Optimazation for Discharge-Excited High-Energy Excimer Laser, AIP Conf. Proceed. No. 100, Subseries on Opt. Sci, & Engin. No. 3, Excimer Lasers 1983 (OSA, Lake Tahoe, Nev. 1983) 33. It is therefore necessary to prepulse the electrodes with a high-voltage, low-energy pulse to initiate the discharge in a timely fashion.
These so called "prepulse techniques" were developed in 1981 (See the two Rothe references, supra, and W. H. Long, M. J. Plummer, E. A. Stappaerts, Efficient Discharge Pumping of an XeCl Laser Using a High-Voltage Prepulse", Appl. Phys. Lett. 43 (1983) 735) and have since been the subject of various papers (see, in addition, R. S. Taylor, K. E. Leopold, Magnetically Induced Pulser Laser Excitation, Appl. Phys. Lett. 46 (1985) 335) and patents (U.S. Pat. Ser. Nos. 4,534,035 and 4,698,518). The prior art prepulse technique is illustrated schematically in FIG. 1, which shows a PFL being charged from a thyratron-switched pulse-charging unit with possible intermediate pulse compression stages. A magnetic isolation switch allows the high voltage prepulse to be applied to the laser electrodes from a separate prepulser, as described in the first Rothe reference, supra. Variations of this arrangement are known in the art. An arrangement with two laser discharge gaps operated in tandem eliminates the need for the isolation switch, as shown in U.S. Pat. Ser. No. 4,534,035. Another arrangement employs a saturable magnetic pulse transformer, which allows the prepulse to be coupled to one electrode through this transformer by magnetic induction. (See U.S. Pat. Ser. No. 4,698,518 and R. S. Taylor, K. E. Leopold, Magnetically Induced Pulser Laser Excitation, Appl. Phys. Lett. 46 (1985) 335).
In order to produce the necessary fast voltage and current risetimes, the prepulse generator must have a low output impedance of less than 10 ohms and must generate a fast voltage pulse with less than 50 nanoseconds risetime. In order to apply the prepulse effectively to one of the laser electrodes of FIG. 1, it is necessary to temporarily isolate the electrode from the PFL by means of a saturable magnetic inductor. This inductor (L.sub.3) becomes partially saturated during activation of the PFL (i.e. during the charging of the PFL capacitance), and then is driven into full saturation by the prepulse. It should be noted that when using a separate prepulser, the prepulse must be of opposite polarity to the voltage supplied by the PFL, so that the magnetic switch L.sub.3 becomes saturated for current flowing from the PFL to the discharge. The resulting field reversal in the discharge gap does not appear to be a problem and may even enhance the discharge stability. (See R. S. Taylor, Preionization and Discharge Stability Study of Long Optical Pulse Duration UV-Preionized XeCl Lasers, Appl. Phys. B (Springer) 41 (1986) 1). Peak voltage requirements for the prepulse are 4V.sub.D +V*.sub.E, where V*.sub.E is the electrode voltage prior to application of the prepulse. In the prior art, the prepulse amplitude, therefore, had to be 100 kV or above, a condition which placed high demands on the low-impedance prepulse generator illustrated in FIG. 1.
Design of the magnetic isolation switch L.sub.3 is difficult because it must have an extremely low saturated inductance of approximately 10 nanohenries. This inductance adds to the head inductance and thus controls the rate of current rise in the discharge. For a PFL output impedance of approximately 1 ohm, the combined inductance of the laser head and the magnetic isolation switch should not be much more than 10 nanohenries, or the benefits of a pulse-forming line are lost. Inductor L.sub.3 should consist of a "racetrack" core around a wide current sheet (single turn). It must be made of a low-loss, high-frequency material to hold off the fast-rising prepulse voltage (&gt;5 MHz). Annealed 15-micrometer thin Metglas ribbon (2705-MN or 2605-CO by Allied Signal Corporation) is a possible choice.
An alternative approach to the prepulse isolation problem is to replace the magnetic isolation switch L.sub.3 with an auxilliary discharge by employing a three-electrode structure as described by D. E. Rothe, Widely Tunable Gas Laser for Remote Sensing of Stratospheric Constituents, Final Tech. Report (Rothe Technical Research Rept. No. 1), NASA-SBIR Contract No. NAS7-935, NASA/JPL (June, 1985).